1. Field of the Invention
The present disclosure relates to a liquid crystal display device and a manufacturing method of the same. Especially, the present disclosure relates to a liquid crystal display device having a protection element for semiconductor layer from externally intruding light and a manufacturing method of the same.
2. Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), and electroluminescence device (or ED).
The liquid crystal display device represents video data by controlling the light transmitivity of the liquid crystal layer using the electric fields. According to the direction of the electric field, the LCD can be classified in the two major types; one is vertical electric field type and the other is the horizontal electric field type.
For the vertical electric field type LCD, the common electrode formed on the upper substrate and the pixel electrode formed on the lower substrate are facing with each other for forming the electric field of which direction is perpendicular to the substrate face. The twisted nematic liquid crystal layer disposed between the upper substrate and the lower substrate is driven the vertical electric field. The vertical electric field type LCD has merit of higher aperture ratio, while it has demerit of narrower view angle about 90 degree.
For the horizontal electric field type LCD, the common electrode and the pixel electrode are formed on the same substrate in parallel. The liquid crystal layer disposed between the upper substrate and the lower substrate is driven in In-Plane-Switching (IPS) mode by the electric field parallel to the substrate face. The horizontal electric field type LCD has a merit of wider view angle over 160 degree.
Hereinafter, we explain about the horizontal electric field type LCD in detail. FIG. 1 is a plane view illustrating the horizontal electric field type LCD according to the related art. FIG. 2 is a cross-sectional view cutting along the line I-I′ for illustrating the structure of the horizontal electric field type LCD of the FIG. 1.
Referring to FIGS. 1 and 2, the horizontal electric field type LCD comprises a thin film transistor (or TFT) array substrate TFTS having the thin film transistor TFTC, a color filter array substrate CFS having the color filter CF and the black matrix BM, and a liquid crystal layer LC disposed the substrates (TFTS and CFS). The TFT array substrate of the horizontal electric field type LCD includes the gate line GLC and the data line DLC crossing each other on the lower substrate SUBLC, the thin film transistor TFTC formed at each crossing point of the gate line GLC and the data line DLC, the pixel electrode PXLC and the common electrode COMC forming the horizontal electric field within the pixel area defined by the crossing structure of the gate line GLC and the data line DLC, and the common line CLC connecting the common electrode COMC.
The thin film transistor TFTC includes the gate electrode GC branching from the gate line GLC, the semiconductor layer AC overlapping with the gate electrode GC on the gate insulating layer GIC covering the gate electrode GC, the source electrode SC branching from the data line DLC and contacting one side of the semiconductor layer AC, and the drain electrode DC facing with the source electrode SC and contacting the other side of the semiconductor layer AC. On the thin film transistor TFTC, the passivation layer PAS SIC is formed to protect the thin film transistor TFTC. The pixel electrode PXLC is formed on the passivation layer PASSIC.
The gate line GLC supplies the gate signal to the gate electrode GC of the thin film transistor TFTC. The data line DLC supplies the pixel signal to the pixel electrode PXLC via the drain electrode DC of the thin film transistor TFTC. The gate line GLC and the data line DLC are crossed each other so they define the pixel area. The common line CLC which is disposed between each pixel area and parallel with the gate line GLC, supplies the reference voltage for driving the liquid crystal layer.
By responding to the gate signal of the gate line GLC, the thin film transistor TFTC turns on to supply the pixel signal from the data line DLC to the pixel electrode PXLC. The pixel electrode PXLC is connected to the drain electrode DC of the thin film transistor TFTC and formed within the pixel area. The common electrode COMC is connected to the common line CLC and formed within the pixel area. Especially, the pixel electrode PXLC and the common electrode COMC are parallel each other in the pixel area. To do this, the common electrode COMC has a plurality of segments which are disposed in parallel and spaced away from each other with predetermined distance, and the pixel electrode PXLC has a plurality of segments and each of them is disposed between the segments of the common electrode COMC.
Therefore, the horizontal electric field is formed between the pixel electrode PXLC on which the pixel signal is applied via the thin film transistor TFTC and the common electrode COMC on which the reference signal is applied via the common line CLC. Especially, the horizontal electric field is formed between the segment of the pixel electrode PXLC and the segment of the common electrode COMC.
By the horizontal electric field, the liquid crystal moleculars arrayed in parallel direction between the thin film transistor array substrate TFTS and the color filter array substrate CFS can be rotated according to the dielectric anisotropy property. According to the rotating state, the light transmissivity through the pixel area will be varied and then the video data can be represented.
The liquid crystal display device as mentioned above uses back light BL radiated from the back light system (not shown) placed under the low substrate SUBLC of the LCD panel for representing the video data. Under this condition, the light entering into the thin film transistor array substrate TFTS may be blocked by the gate electrode GC and it may enter into the semiconductor layer AC by diffraction phenomena. Some back light BL entering into the pixel area and considered as not affecting to the semiconductor layer AC may intrude into the semiconductor layer AC exposed between the source electrode SC and the drain electrode DC by reflecting from the black matrix BM formed on the color filter array substrate CFS or the upper substrate SUBLC.
Like this, if the light is induced into the semiconductor layer AC of the thin film transistor TFTC, the characteristic curve of the thin film transistor TFTC will be shifted one side (especially, left) due to the optical and thermal stresses so that the property and performance of the thin film transistor TFTC may be degraded. FIG. 3 is a graph showing the shift phenomenon of the characteristic curve of the thin film transistor when the light enters into the TFT. As a result, the current leakage may be occurred and the thin film transistor TFTC may not transmit the pixel signal to the pixel electrode PXLC normally.